Fans believe immortality is a factor that usually makes a character hard to write, and one Naruto character fits this description perfectly. However, technically, he is not entirely immortal but could be considered unkillable because of a condition of his ability.
The character was a part of Akatsuki and was teamed up with a pure immortal character named Hidan. Hidan was a true believer of Lord Jashin and likely the reason behind his immortality. Surprisingly, he also uses this ability in combat by hitting his opponent and himself, which could, in turn, affect his opponent. He killed Asuma Sarutobi, the son of the Third Hokage, using this ability.
Just like his partner, Kakuzu was also as dangerous on the battlefield and was really close to taking down Kakashi, the instructor of Team 7. The technique that made Kakuzu conditionally immortal was his forbidden technique, which gave him the ability to store hearts, with each heart making one life's worth for Kakuzu.
Naruto character analysis: Kakuzu and the technique behind his near invulnerability, explained
Kakuzu is a Naruto character who wielded the forbidden technique named Earth Grudge Fear that made its user's body similar to that of a rag-doll, such that their whole body would be connected by strings. He stole this technique from his home village, the Village Hidden in the Waterfall.
This technique also allowed Kakuzu to store the hearts of his opponents as his own heart, which made him a conditionally immortal Naruto character. He could store four hearts other than his original one, and as soon as one would be destroyed, replacements are already present for him.
Kakuzu is a member of the S-rank group of criminals, the Akatsuki. The Naruto character was introduced in episode 71 of Naruto Shippuden. He was always seen with his duo partner, Hidan. Kakuzu had a very eerie appearance due to which his face would always be covered.
From the very start of the series, Kakuzu has been an elite ninja and was assigned to assassinate the God of Shinobi, Hashirama Senju. As predicted, he lost and returned to this village. Sometime after, Kakuzu was put into a team of four people that later became a part of Kakuzu due to his technique, the Earth Grudge Fear.
The Earth Grudge Fear was the priced possession of Kakuzu's village, the Hidden Waterfall Village, but the S-rank shinobi stole it and used it to steal the hearts of his four partners. He absorbed each heart into his body, thus creating five hearts for his body and each heart acted as a life for him. These masks also helped him during fights.
During his fight with the Hidden Leaf shinobi, his hearts were destroyed one by one by his opponents. The first one was destroyed by Kakashi as Hidden Leaf shinobi went solo against the Akatsuki member.
The second was destroyed due to Shikamaru's strategy, as he faked Hidan into thinking that he had his blood, whereas, in reality, he made Hidan lick Kakuzu's blood, thus leaving two hearts behind.
After this, Kakuzu fought the Hidden Leaf Shinobi with the intent to kill. Fortunately, Naruto made a last-time appearance, and after a first failed try, landed his Rasenshuriken on Kakuzu. Now, Kakuzu was left with no heart as he lay motionless on the ground. Before returning to the village, Kakashi landed the last blow on him, thereby killing him.
Kakuzu was later revived during the Fourth Great Ninja War by Kabuto. But following his demise, this time his opponents were ready for anything and he didn't get to enjoy the spotlight like he did during his first time.
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